What is a Solar Flare?

Solar flares and the often associated coronal mass ejections
are the biggest explosions in today's solar system. Sometimes they have as much energy as
a billion megatons of TNT, several orders of magnitude more than the energy of the recent
crash of Comet Shoemaker-Levy into Jupiter. This tremendous amount of energy is typically
released in only a few minutes. Plasma is heated to tens of millions of degrees and
electrons, protons, and heavy nuclei are accelerated to near the speed of light. Some of
these particles stop in the solar atmosphere and produce the observed X-ray and gamma-ray
emissions. Other particles travel outward from the Sun and are detected in the vicinity of
the Earth and beyond.

Where Does the Energy Come From?

It is believed that the energy of a solar flare comes originally from the violent
motions below the solar surface. Prior to a flare, it is thought that energy builds up
from below and is stored in the magnetic field that pervades the solar atmosphere. This
usually occurs near a sunspot, but exactly how it happens and what triggers it to be
suddenly and explosively released is not known.

Why Study Solar Flares?

Because they are such powerful explosions that are little understood and cannot be
predicted.

Because they can have dangerous and disruptive consequences in space and on the Earth.

Because the Sun is so much closer and more easily observed than the other sites in the
universe -- such as pulsars, quasars, and black holes -- where the same high-energy
processes are believed to be occurring.

Why Study Solar Flares Now?

The occurrence rate of solar flares varies on an 11-year cycle as
illustrated below, where the number of flares recorded per month is
plotted versus time for the last two cycles. The dotted line shows the
predicted rate for the current cycle based on a simple average of the
previous two cycles. Most flares, particularly the rare, large bursts
with the most dangerous effects, occur within two to three years of the
peak in the 11-year cycle. HESSI will take full advantage of the current
peak in the solar activity cycle with up to three years of flight operations
beginning in the year 2001.

What Does a Solar Flare Look Like?

The picture of the Sun that is shown here was taken
with the Soft X-Ray Telescope on the joint Japan-U.S. Yohkoh
mission. It shows the solar atmosphere at a temperature of about 2 million degrees glowing
in soft X-rays. The diagram on the right is a
blow-up of a flare, inside the blue square, as seen with Yohkoh in soft and hard X-rays.
The soft X-rays shown in color are from plasma heated during the flare to temperatures of
10-20 million degrees. The plasma is constrained to the loop-like shape by the magnetic
field shown schematically by the blue lines. The hard X-ray sources shown by the white
contour lines are from the electrons accelerated during the flare. The possible site of
the original energy release is indicated high in the corona.

How are X-rays Produced in Solar Flares?

The X-rays are believed to be produced by the electrons accelerated in the solar corona
during the flare. As the electrons travel at velocities about one third the speed of light
in the corona, a small fraction of them (1 in 100,000) suffer close encounters with the
ambient protons, as indicated below. In such an
interaction, the electron is attracted towards the proton as a result of the opposite
charges, and its path is bent. An X-ray photon is produced at the point of the electron's
closest approach to the proton. This is known as bremsstrahlung, from the German word
meaning braking radiation. By detecting these X-ray photons with HESSI, scientists will be
able to determine where and how many electrons are accelerated and to what energies.

How are Gamma Rays Produced During Solar Flares?

Just as electrons are accelerated during solar flares, free protons and the nuclei of
heavier elements in the solar atmosphere are also accelerated. Some accelerated protons
encounter the nuclei of carbon, oxygen, neon and other elements found in the solar
atmosphere. When a proton collides with one of these nuclei, the nucleus is excited to a
higher energy level. The excited nucleus gives off a gamma-ray photon with a specific
energy characteristic of the element involved and returns to its original energy level or
ground state. Alternatively, an accelerated heavy nucleus can interact with an ambient
low-energy proton and become excited to a higher energy level. It continues on at a
similar velocity and emits the characteristic gamma ray as it decays back to the ground
state. Because of the velocity of the heavy nucleus, the gamma-ray energy is Doppler
shifted up or down, depending on whether the nucleus is moving towards or away from the
observer, respectively. The process is illustrated below.